Alkaline dry batteries are used as a power source for electronic devices such as digital still cameras, and various attempts have been made to improve the heavy load discharge characteristic of alkaline dry batteries. Examples of such attempts include improvement of active materials and optimization of the amounts of filled active materials for heightening reaction efficiency and reduction of internal resistance, and thinning the components of alkaline dry batteries for increasing the internal volume thereof.
In addition, to improve the heavy load discharge characteristic, heightening the current collection efficiency of the gelled negative electrode has also been examined. For example, Japanese Laid-Open Patent Publication No. 2004-95475 proposes the use of a negative electrode current collector that is shaped like a plate, a spiral or a bundle of thin fibers, or a negative electrode current collector composed of a combination of a plurality of such shapes, in order to increase the current collection area.
In commercially-available, AA-size cylindrical alkaline dry batteries, the diameter of the body of the negative electrode current collector (the columnar portion inserted into the gelled negative electrode) is 1.3 to 1.5 mm, the height of the gelled negative electrode filled (the length in the axial direction of the cylindrical alkaline dry battery) is approximately 42 mm, and the length of the portion of the body of the negative electrode current collector inserted into the gelled negative electrode is approximately 65% of the height of the gelled negative electrode filled.
Japanese Laid-Open Patent Publication No. 2002-190303 proposes setting the length of the portion of the negative electrode current collector inserted into the gelled negative electrode to a dimension equivalent to the height of the filled gelled negative electrode, in order to maximize the current collecting efficiency.
However, such negative electrode current collectors are difficult to produce or work and require high production costs, so their practical utilization is difficult. The effect of the surface area (current collection area) of the negative electrode current collector on the discharge characteristics and the effect of the length of the portion of the negative electrode current collector inserted into the gelled negative electrode on the heavy load discharge characteristic differ according to the size of the alkaline dry battery such as the D-size or AA-size. The heavy load discharge characteristic is believed to be dependent on the length of the gelled negative electrode in the axial direction of the cylindrical alkaline dry battery.
As in Japanese Laid-Open Patent Publication No. 2004-95475, when the shape of the negative electrode current collector is changed to increase the current collection area of the negative electrode current collector, the volume of the negative electrode current collector may increase and the volume of the filled gelled negative electrode may decrease in a corresponding amount, thereby resulting in a reduction in battery capacity. Thus, the greatest care should be given to the balance between the current collecting efficiency and the amount of active material.
In order to solve the conventional art problems discussed above, it is an object of the invention to provide an alkaline dry battery having excellent heavy load discharge characteristic while having a simple and inexpensive structure.